The disclosure of Japanese Patent Application No. 2000-151602 filed on May 23, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to a vehicle drive power control apparatus which determines a target drive power based on a state of operation of a vehicle driven by output of an internal combustion engine via a continuously variable transmission, and which controls the torque of the engine and the speed ratio of the continuously variable transmission so as to obtain an output of the engine for achieving the target drive power. The invention also relates to a control method of the apparatus.
2. Description of Related Art
As apparatuses for controlling the drive power of a vehicle so as to achieve good fuel economy, apparatuses of generally termed coordinate control performed through the use of a continuously variable transmission are known (Japanese Patent Application Laid-Open No. 11-198684 and No. 10-329587). This coordinate control determines a target drive power based on the state of operation of the vehicle, and coordinately controls the torque of the internal combustion engine and the speed ratio of the continuously variable transmission so as to obtain the engine output that achieves the determined target drive power with a minimum fuel consumption rate. Through the coordinate control, the fuel economy is improved.
In such a drive power control apparatus, speed shift lines of the continuously variable transmission are set so as to conform to optimal fuel economy lines (FIG. 19) based on the efficiency of the internal combustion engine, or to optimal fuel economy lines (comparative examples indicated by one-dot chain lines in FIGS. 12 and 15) determined with the efficiency of the internal combustion engine and the efficiency of the continuously variable transmission (FIG. 20) taken into consideration.
However, in the vehicles equipped with the above-described drive power control apparatus, the fuel economy has not been sufficiently improved in a practical region. A reason for the insufficient improvement is as follows. Based on the speed shift lines set so as to conform to optimal fuel economy lines as described above, the running of the vehicle in a practical region involves a fuel consumption increase corresponding to the inertia torques caused by fluctuated rotations of an input shaft of the continuously variable transmission, so that a low efficiency results as a whole.
It is an object of the invention to provide a vehicle drive power control apparatus capable of improving fuel economy by reducing the fuel consumption caused by the aforementioned inertia torque and thereby enhancing the efficiency as a whole, and a control method of the apparatus.
In accordance with a first mode of the invention, a drive power control apparatus of a vehicle that is driven by the output of an engine via a transmission determines a target drive power based on a state of operation of the vehicle, and controls the torque of the engine and the speed ratio of the transmission so as to provide an output of the engine for achieving the target drive power. The control apparatus controls the speed ratio of the transmission based on a speed shift line that is plotted on a graph having a pair of axis defining the revolution speed of the engine and the torque of the engine so that, within a practical region, the speed shift line is at a low revolution speed side of an optimal fuel consumption line that is determined based on at least the efficiency of the engine and the efficiency of the transmission, of the efficiency of a drive system that includes the engine and the transmission.
The speed ratio of the transmission is controlled in accordance with the speed shift line that is set so that, within the practical region, the speed shift line is located on the low revolution speed side of the optimal fuel consumption line determined based on the efficiencies of the engine and the transmission of the drive system. Thus, the speed shift line is shifted to the low engine speed side within the practical region. Therefore, the width of increase in engine revolution speed from the level occurring at the beginning of the practical region is curbed. Hence, the fuel consumption resulting from inertia torques caused by fluctuations in the engine revolution speed, that is, fluctuations in the revolution speed of the input shaft of the transmission, is reduced, so that the efficiency as a whole increases and the fuel economy improves in comparison with the case where the optimal fuel consumption line is used as a control basis.
The speed shift line may be set so that, within the practical region, a difference between a minimum revolution speed and a maximum revolution speed on the speed shift line is smaller than a difference between a minimum revolution speed and a maximum revolution speed on the optimal fuel consumption line. Furthermore, the speed shift line may be set so that, within the practical region, a sensitivity of a fluctuation in the revolution speed with respect to a fluctuation in the target drive power on the speed shift line is lower than a sensitivity of a fluctuation in the revolution speed with respect to the fluctuation in the target drive power on the optimal fuel consumption line.
The aforementioned construction prevents great fluctuations in the revolution speed of the engine even if the output of the engine fluctuates in accordance with the target drive power within the practical region. Therefore, the fuel consumption resulting from inertia torques caused by fluctuations in the engine revolution speed, that is, fluctuations in the revolution speed of the input shaft of the transmission, is reduced, so that the efficiency as a whole increases and the fuel economy improves in comparison with the case where the optimal fuel consumption line is used as a control basis.
The drive power control apparatus may further have a construction wherein the engine comprises a NOx storage-reduction type catalyst in an exhaust system, and during the lean combustion, the engine reduces NOx stored in the NOx storage-reduction type catalyst by performing a rich spike control of temporarily changing an air-fuel mixture so that a fuel concentration in the mixture becomes higher than the fuel concentration corresponding to a stoichiometric air-fuel ratio, and wherein on a boundary line between the lean combustion and the stoichiometric air-fuel ratio combustion in the two-dimensional space of the revolution speed of the engine and the torque of the engine, the speed shift line passes through or near a point at which a corrected fuel consumption rate determined by considering the rich spike control and a fuel consumption rate provided during the lean combustion becomes equal to or closest to a fuel consumption rate provided during the stoichiometric air-fuel ratio combustion.
If the selectable forms of combustion includes a stoichiometric air-fuel ratio combustion and a lean combustion and the rich spike control is performed during the lean combustion, the speed shift line is set so that the point which exists on the boundary line between the lean combustion and the stoichiometric air-fuel ratio combustion and through which the speed shift line passes coincides with or exists near the point at which the corrected fuel consumption rate determined by considering the rich spike control and the fuel consumption rate provided during the lean combustion becomes equal to or closest to the fuel consumption rate provided during the stoichiometric air-fuel ratio combustion. This makes it possible to maintain a state of good fuel consumption rate even when the form of combustion changes between the lean combustion and the stoichiometric air-fuel ratio combustion in accordance with the speed shift line. Thus, the changing between the forms of combustion is optimized, so that fuel economy can be further improved.
In a vehicle drive control method in accordance with another mode of the invention, a target output of the engine for achieving a target drive power set based on a state of operation of the vehicle is calculated. Furthermore, a target revolution speed of the engine is set based on a speed shift line that is plotted on a graph having a pair of axis defining a revolution speed of the engine and the target output of the engine so that, within a practical region in which the state of operation of the engine is practical, a width of increase in the revolution speed of the engine to a relatively high revolution speed on the speed shift line is curbed compared with an optimal fuel consumption line determined based on at least an efficiency of the engine and an efficiency of the transmission, of an efficiency of a drive system that includes the engine and the transmission. Then, the speed ratio of the transmission is controlled so that an actual revolution speed of the engine becomes equal to the target revolution speed.
According to this mode of the invention, the fuel consumption resulting from inertia torques caused by fluctuations in the engine revolution speed, that is, fluctuations in the revolution speed of the input shaft of the transmission, is reduced, so that the efficiency as a whole increases and the fuel economy improves in comparison with the case where the optimal fuel consumption line is used as a control basis.